Impact of electric charge and motion of water drops on the inception field strength of partial discharges

Strong electric fields may deform drops and induce their oscillation or motion on the substrate. Moreover, they can initiate partial discharges (PDs) because of the enhancement of the electric field in the vicinity of the three-phase contact lines. The partial discharges affect the drop spreading which can result in unusual drop shapes. In addition, the partial discharges can also deteriorate the surface properties of the substrate, e.g., of high-voltage composite insulators. In this study the occurrence of partial discharges due to stationary or oscillating sessile drops under the influence of an alternating electric field is investigated using a generic insulator model under well-defined conditions. Drops of a yield stress fluid (a gelatin-water mixture) are used to determine the PD inception field strength for stationary drop shapes. The influence of the volume as well as the distance between the individual drops for two drop configurations on the PD inception threshold is determined. The inception field strength of the partial discharges is measured for various drop volumes, drop charges, as well as for different resonance modes of drop oscillations. Besides the electrical measurement, the location of the partial discharges is optically determined by a UV camera. The detailed knowledge of the influencing factors of the partial discharges improves the understanding of the drop behavior under the impact of strong electric fields.

Figure 1

Schematic of the specimen, reprinted figure with permission from [29].

Figure 2

Schematic of the experimental setup. The UV camera is used either in top or in side view.

Figure 3

Schematic of the drop charger, reprinted with permission from [29].

Figure 4

Frequency-dependent relative permittivity of water and gelatin at different temperatures $\vartheta$.

Figure 5

Exemplary images of gelatin drops: (a) $V\approx 65 \mu$l and (b) $V\approx 130 \mu$l with superimposed image of UV camera to visualize the partial discharges (colored in magenta) in (a) side view for $\widehat{E}=12.48$ kV/cm and (b) top view for $\widehat{E}=10.27$ kV/cm.

Figure 6

Exemplary images of water drops ($V=20\mu \mathrm{l}$) with superimposed image of UV camera to visualize the partial discharges (colored in magenta) in (a) side view for $\widehat{E}=9.02$ kV/cm and (b) top view for $\widehat{E}=9.63$ kV/cm.

Figure 7

Partial discharge inception field strength ${\widehat{E}}_{\mathrm{in}}$ ($f=50$ Hz) for single gelatin drops depending on their volume on silicone rubber and epoxy. The error bars show the standard deviation of the mean value.

Figure 8

Exemplary images of two gelatin drops: (a) $V\approx 60 \mu$l and (b) $V\approx 150 \mu$l with superimposed image of UV camera to visualize the partial discharges (colored in magenta) in (a) side view for $\widehat{E}\approx 9.9$ kV/cm and (b) top view for $\widehat{E}\approx 10.1$ kV/cm.

Figure 9

Partial discharge inception field strength ${\widehat{E}}_{\mathrm{in}}$ of two gelatin drops depending on the distance for different volumes. The error bars show the standard deviation of the mean value.

Figure 10

One cycle of the first three modes for $n=2,3,4$ of uncharged drops. (a) Example of mode 1 ($n=2$) of a 20-$\mu \mathrm{l}$ drop at 27 Hz and $\widehat{E}=3.81\mathrm{kV}$/cm, (b) example of mode 2 ($n=3$) of a 30-$\mu \mathrm{l}$ drop at 23 Hz and $\widehat{E}=4.67\mathrm{kV}$/cm and (c) example of mode 3 ($n=4$) of a 60-$\mu \mathrm{l}$ drop at 48 Hz and $\widehat{E}=7.37\mathrm{kV}$/cm. Reprinted figure with permission from [29].

Figure 11

Partial discharge inception field strength ${\widehat{E}}_{\mathrm{in}}$ of single uncharged water drops depending on the volume and resonance mode.

Figure 12

Partial discharge inception field strength ${\widehat{E}}_{\mathrm{in}}$ of single water drops depending on the volume and electric charge for oscillation mode 1. The error bars show the standard deviation of the mean value.

Figure 13

Partial discharge inception field strength ${\widehat{E}}_{\mathrm{in}}$ of single water drops depending on the volume and electric charge for oscillation mode 2. The error bars show the standard deviation of the mean value.